They were never a threat as a country. Now that the place has fallen into anarchy, they've become much more of a breeding ground for terrorism. The US had better keep the borders on lock-down for a good long time. If they leave now, thousands of Iraqis will be leaving the country to carry out terrorist acts.
The US should have just used the CIA to spark a civil war, revolution or coup. Although that's illegal under international treaties, it would have kept the US out of that quagmire, at least officially.
